practice problems

1. compare a molecule of oxygen (o₂) and a molecule of water (h₂o).

2. which is a compound?
a) o₂
b) co₂
c) he
d) h

3. fill in the blank: an element is made of only one kind of ______.

4. how many atoms of each element are in the formula c₆h₁₂o₆?

5. use the periodic table: what element has the symbol na?

6. in the formula h₂o, how many oxygen atoms are there?

7. is ice melting a physical or chemical change? why?

8. baking a cake is a ______ change.

9. which is evidence of a chemical change?
a) change in shape
b) formation of bubbles
c) breaking into pieces
d) dissolving

10. in a saltwater solution, which is the solute? which is the solvent?

11. what happens to concentration if you add more water to a sugary drink?

Question

practice problems

1. compare a molecule of oxygen (o₂) and a molecule of water (h₂o).

2. which is a compound?
   a) o₂
   b) co₂
   c) he
   d) h

3. fill in the blank: an element is made of only one kind of ______.

4. how many atoms of each element are in the formula c₆h₁₂o₆?

5. use the periodic table: what element has the symbol na?

6. in the formula h₂o, how many oxygen atoms are there?

7. is ice melting a physical or chemical change? why?

8. baking a cake is a ______ change.

9. which is evidence of a chemical change?
   a) change in shape
   b) formation of bubbles
   c) breaking into pieces
   d) dissolving

10. in a saltwater solution, which is the solute? which is the solvent?

11. what happens to concentration if you add more water to a sugary drink?

Accepted Answer

Let's solve these problems one by one:

### Problem 1: Compare a molecule of oxygen ($O_2$) and a molecule of water ($H_2O$)
# Explanation:
## Step 1: Identify the composition of $O_2$
Oxygen molecule ($O_2$) is made up of two oxygen atoms. It is a diatomic molecule of the element oxygen.
## Step 2: Identify the composition of $H_2O$
Water molecule ($H_2O$) is made up of two hydrogen atoms and one oxygen atom. It is a compound as it contains atoms of different elements (hydrogen and oxygen) chemically bonded together.
## Step 3: Compare the two molecules
- **Type of substance**: $O_2$ is an element (a diatomic molecule of the element oxygen), while $H_2O$ is a compound (contains two different elements).
- **Atomic composition**: $O_2$ has only oxygen atoms, $H_2O$ has hydrogen and oxygen atoms.
- **Properties**: Oxygen is a gas that supports combustion, water is a liquid (at room temperature) that is essential for life and does not support combustion in the same way.

### Problem 2: Which is a compound?
Options: A) $O_2$, B) $CO_2$, C) $He$, D) $H$
# Explanation:
## Step 1: Recall the definition of a compound
A compound is a substance made up of two or more different elements chemically bonded together.
## Step 2: Analyze each option
- **Option A ($O_2$)**: Made up of only oxygen atoms, so it is an element (diatomic molecule of oxygen).
- **Option B ($CO_2$)**: Made up of carbon and oxygen atoms (two different elements) chemically bonded, so it is a compound.
- **Option C ($He$)**: Helium is a noble gas, made up of only helium atoms, so it is an element.
- **Option D ($H$)**: Represents a hydrogen atom or the element hydrogen (in atomic form), so it is an element.
# Answer: B) $CO_2$

### Problem 3: Fill in the blank: An element is made of only one kind of ______.
# Explanation:
## Step 1: Recall the definition of an element
An element is a pure substance that cannot be broken down into simpler substances by chemical means. It is made up of only one kind of atom. For example, all atoms in a sample of oxygen (element) are oxygen atoms.
# Answer: atom

### Problem 4: How many atoms of each element are in the formula $C_6H_{12}O_6$?
# Explanation:
## Step 1: Recall how to read chemical formulas
In a chemical formula, the subscript after an element symbol indicates the number of atoms of that element in the molecule. If there is no subscript, it means there is 1 atom.
## Step 2: Analyze $C_6H_{12}O_6$
- For carbon (C): The subscript is 6, so there are 6 carbon atoms.
- For hydrogen (H): The subscript is 12, so there are 12 hydrogen atoms.
- For oxygen (O): The subscript is 6, so there are 6 oxygen atoms.
# Answer: Carbon: 6 atoms, Hydrogen: 12 atoms, Oxygen: 6 atoms

### Problem 5: Use the Periodic Table: What element has the symbol Na?
# Explanation:
## Step 1: Recall the Periodic Table symbols
From the Periodic Table, the symbol Na is for the element Sodium.
# Answer: Sodium

### Problem 6: In the formula $H_2O$, how many oxygen atoms are there?
# Explanation:
## Step 1: Analyze the formula $H_2O$
In the chemical formula $H_2O$, the subscript after O (oxygen) is 1 (since there is no written subscript, the default is 1). Wait, no: Wait, $H_2O$ has 2 hydrogen atoms and 1 oxygen atom. Wait, the subscript for O is 1? Wait, no, the formula is $H_2O$, so the number of oxygen atoms is 1? Wait, no, let's check again. The formula $H_2O$: H has subscript 2, O has subscript 1 (implicit). So there is 1 oxygen atom.
Wait, no, maybe I made a mistake. Wait, $H_2O$: the subscript for O is 1? Wait, no, the formula is written as $H_2O$, which means 2 hydrogen atoms and 1 oxygen atom. So the number of oxygen atoms is 1.
Wait, but let's confirm. In a chemical formula, the subscript indicates the number of atoms of the preceding element. So in $H_2O$, H has 2 atoms, O has 1 atom (since there's no subscript, it's 1). So the number of oxygen atoms is 1.
# Answer: 1

### Problem 7: Is ice melting a physical or chemical change? Why?
# Explanation:
## Step 1: Recall the definitions of physical and chemical changes
- **Physical change**: A change in which the substance changes its physical state (like solid to liquid, liquid to gas, etc.) but not its chemical composition. No new substance is formed.
- **Chemical change**: A change in which a new substance is formed with different chemical properties.
## Step 2: Analyze ice melting
When ice (solid water) melts, it changes from solid to liquid water. The chemical composition remains $H_2O$ (water molecules are still the same). No new substance is formed. So it is a physical change.
# Answer: Physical change. Because ice (solid water) melts to form liquid water, and the chemical composition (H₂O) remains the same. No new substance is formed.

### Problem 8: Baking a cake is a ______ change.
# Explanation:
## Step 1: Analyze the process of baking a cake
When baking a cake, ingredients like flour, eggs, sugar, etc., undergo chemical reactions (like proteins denaturing, starches gelatinizing, sugars caramelizing, etc.). New substances are formed (like the cake structure, flavors from reactions). So it is a chemical change.
# Answer: chemical

### Problem 9: Which is evidence of a chemical change?
Options: A) Change in shape, B) Formation of bubbles, C) Breaking into pieces, D) Dissolving
# Explanation:
## Step 1: Recall evidence of chemical change
Evidence of chemical change includes: formation of a gas (bubbles, if from a chemical reaction), formation of a precipitate, change in color, change in temperature, production of light or odor.
## Step 2: Analyze each option
- **Option A (Change in shape)**: This is a physical change (like bending a metal, cutting a paper). No new substance is formed.
- **Option B (Formation of bubbles)**: If bubbles are formed due to a chemical reaction (like baking soda reacting with vinegar to produce $CO_2$ gas), this is evidence of a chemical change (gas formation).
- **Option C (Breaking into pieces)**: This is a physical change (like breaking a glass). No new substance is formed.
- **Option D (Dissolving)**: This is a physical change (like salt dissolving in water, the salt is still $NaCl$). No new substance is formed (in most cases, dissolving is physical, unless it's a reaction like dissolving zinc in acid, but the option just says "dissolving" generally).
# Answer: B) Formation of bubbles

### Problem 10: In a saltwater solution, which is the solute? Which is the solvent?
# Explanation:
## Step 1: Recall definitions of solute and solvent
- **Solute**: The substance that is dissolved in a solution.
- **Solvent**: The substance that dissolves the solute, usually present in a larger amount.
## Step 2: Analyze saltwater solution
In saltwater, salt (usually $NaCl$) is dissolved in water. So salt is the solute (the substance being dissolved), and water is the solvent (the substance doing the dissolving).
# Answer: Solute: Salt (NaCl); Solvent: Water

### Problem 11: What happens to concentration if you add more water to a sugary drink?
# Explanation:
## Step 1: Recall the definition of concentration
Concentration is the amount of solute (sugar, in this case) per unit volume of solvent (water, in this case).
## Step 2: Analyze the effect of adding more water
If you add more water (solvent) to the sugary drink (which has a certain amount of sugar, the solute), the total volume of the solution increases, but the amount of sugar (solute) remains the same. Concentration (e.g., mass/volume or moles/volume) is calculated as $	ext{Concentration} = \frac{	ext{Amount of Solute}}{	ext{Volume of Solution}}$. Since the amount of solute is constant and the volume of solution increases (because we added more water), the concentration will decrease.
# Answer: The concentration decreases. Because concentration is the amount of solute per unit volume of solution. Adding more water (solvent) increases the total volume of the solution while the amount of solute (sugar) remains the same, so the concentration (solute per unit volume) decreases.

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QUESTION IMAGE

Question

Explanation:

Problem 1: Compare a molecule of oxygen ($O_2$) and a molecule of water ($H_2O$)

Step 1: Identify the composition of $O_2$

Step 2: Identify the composition of $H_2O$

Step 3: Compare the two molecules

Problem 2: Which is a compound?

Step 1: Recall the definition of a compound

Step 2: Analyze each option

Step 1: Recall the definition of an element

Answer:

Problem 3: Fill in the blank: An element is made of only one kind of ______.